cpython/Doc/ext.tex

\documentstyle[twoside,11pt,myformat]{report}

\title{\bf Extending and Embedding the Python Interpreter}

\author{
	Guido van Rossum \\
	Dept. CST, CWI, Kruislaan 413 \\
	1098 SJ Amsterdam, The Netherlands \\
	E-mail: {\tt guido@cwi.nl}
}

% Tell \index to actually write the .idx file
\makeindex

\begin{document}

\pagenumbering{roman}

\maketitle

\begin{abstract}

\noindent
This document describes how you can extend the Python interpreter with
new modules written in C or C++.  It also describes how to use the
interpreter as a library package from applications using Python as an
``embedded'' language.

\end{abstract}

\pagebreak

{
\parskip = 0mm
\tableofcontents
}

\pagebreak

\pagenumbering{arabic}

\chapter{Extending Python with C or C++ code}

It is quite easy to add non-standard built-in modules to Python, if
you know how to program in C.  A built-in module known to the Python
programmer as foo is generally implemented in a file called
foomodule.c.  The standard built-in modules also adhere to this
convention, and in fact some of them form excellent examples of how to
create an extension.

Extension modules can do two things that can't be done directly in
Python: implement new data types and provide access to system calls or
C library functions.  Since the latter is usually the most important
reason for adding an extension, I'll concentrate on adding "wrappers"
around C library functions; the concrete example uses the wrapper for
system() in module posix, found in (of course) the file posixmodule.c.

It is important not to be impressed by the size and complexity of
the average extension module; much of this is straightforward
"boilerplate" code (starting right with the copyright notice!).

Let's skip the boilerplate and jump right to an interesting function:

\begin{verbatim}
    static object *
    posix_system(self, args)
        object *self;
        object *args;
    {
        char *command;
        int sts;
        if (!getargs(args, "s", &command))
            return NULL;
        sts = system(command);
        return newintobject((long)sts);
    }
\end{verbatim}

This is the prototypical top-level function in an extension module.
It will be called (we'll see later how this is made possible) when the
Python program executes statements like

\begin{verbatim}
    >>> import posix
    >>> sts = posix.system('ls -l')
\end{verbatim}

There is a straightforward translation from the arguments to the call
in Python (here the single value 'ls -l') to the arguments that are
passed to the C function.  The C function always has two parameters,
conventionally named 'self' and 'args'.  In this example, 'self' will
always be a NULL pointer, since this is a function, not a method (this
is done so that the interpreter doesn't have to understand two
different types of C functions).

The 'args' parameter will be a pointer to a Python object, or NULL if
the Python function/method was called without arguments.  It is
necessary to do full argument type checking on each call, since
otherwise the Python user could cause a core dump by passing the wrong
arguments (or no arguments at all).  Because argument checking and
converting arguments to C is such a common task, there's a general
function in the Python interpreter which combines these tasks:
getargs().  It uses a template string to determine both the types of
the Python argument and the types of the C variables into which it
should store the converted values.

When getargs returns nonzero, the argument list has the right type and
its components have been stored in the variables whose addresses are
passed.  When it returns zero, an error has occurred.  In the latter
case it has already raised an appropriate exception by calling
err_setstr(), so the calling function can just return NULL.

The form of the format string is described at the end of this file.
(There are convenience macros getstrarg(), getintarg(), etc., for many
common forms of argument lists.  These are relics from the past; it's
better to call getargs() directly.)


\section{Intermezzo: errors and exceptions}

An important convention throughout the Python interpreter is the
following: when a function fails, it should set an exception condition
and return an error value (often a NULL pointer).  Exceptions are set
in a global variable in the file errors.c; if this variable is NULL no
exception has occurred.  A second variable is the "associated value"
of the exception.

The file errors.h declares a host of err_* functions to set various
types of exceptions.  The most common one is err_setstr() -- its
arguments are an exception object (e.g. RuntimeError -- actually it
can be any string object) and a C string indicating the cause of the
error (this is converted to a string object and stored as the
"associated value" of the exception).  Another useful function is
err_errno(), which only takes an exception argument and constructs the
associated value by inspection of the (UNIX) global variable errno.

You can test non-destructively whether an exception has been set with
err_occurred().  However, most code never calls err_occurred() to see
whether an error occurred or not, but relies on error return values
from the functions it calls instead:

When a function that calls another function detects that the called
function fails, it should return an error value but not set an
condition -- one is already set.  The caller is then supposed to also
return an error indication to *its* caller, again *without* calling
err_setstr(), and so on -- the most detailed cause of the error was
already reported by the function that detected it in the first place.
Once the error has reached Python's interpreter main loop, this aborts
the currently executing Python code and tries to find an exception
handler specified by the Python programmer.

To ignore an exception set by a function call that failed, the
exception condition must be cleared explicitly by calling err_clear().
The only time C code should call err_clear() is if it doesn't want to
pass the error on to the interpreter but wants to handle it completely
by itself (e.g. by trying something else or pretending nothing
happened).

Finally, the function err_get() gives you both error variables
*and clears them*.  Note that even if an error occurred the second one
may be NULL.  I doubt you will need to use this function.

Note that a failing malloc() call must also be turned into an
exception -- the direct caller of malloc() (or realloc()) must call
err_nomem() and return a failure indicator itself.  All the
object-creating functions (newintobject() etc.) already do this, so
only if you call malloc() directly this note is of importance.

Also note that, with the important exception of getargs(), functions
that return an integer status usually use 0 for success and -1 for
failure.

Finally, be careful about cleaning up garbage (making appropriate
[X]DECREF() calls) when you return an error!


\section{Back to the example}

Going back to posix_system, you should now be able to understand this
bit:

\begin{verbatim}
        if (!getargs(args, "s", &command))
            return NULL;
\end{verbatim}

It returns NULL (the error indicator for functions of this kind) if an
error is detected in the argument list, relying on the exception set
by getargs().  The string value of the argument is now copied to the
local variable 'command'.

If a Python function is called with multiple arguments, the argument
list is turned into a tuple.  Python programs can us this feature, for
instance, to explicitly create the tuple containing the arguments
first and make the call later.

The next statement in posix_system is a call tothe C library function
system(), passing it the string we just got from getargs():

\begin{verbatim}
        sts = system(command);
\end{verbatim}

Python strings may contain internal null bytes; but if these occur in
this example the rest of the string will be ignored by system().

Finally, posix.system() must return a value: the integer status
returned by the C library system() function.  This is done by the
function newintobject(), which takes a (long) integer as parameter.

\begin{verbatim}
        return newintobject((long)sts);
\end{verbatim}

(Yes, even integers are represented as objects on the heap in Python!)
If you had a function that returned no useful argument, you would need
this idiom:

\begin{verbatim}
        INCREF(None);
        return None;
\end{verbatim}

'None' is a unique Python object representing 'no value'.  It differs
from NULL, which means 'error' in most contexts (except when passed as
a function argument -- there it means 'no arguments').


\section{The module's function table}

I promised to show how I made the function posix_system() available to
Python programs.  This is shown later in posixmodule.c:

\begin{verbatim}
    static struct methodlist posix_methods[] = {
        ...
        {"system",  posix_system},
        ...
        {NULL,      NULL}        /* Sentinel */
    };

    void
    initposix()
    {
        (void) initmodule("posix", posix_methods);
    }
\end{verbatim}

(The actual initposix() is somewhat more complicated, but most
extension modules are indeed as simple as that.)  When the Python
program first imports module 'posix', initposix() is called, which
calls initmodule() with specific parameters.  This creates a module
object (which is inserted in the table sys.modules under the key
'posix'), and adds built-in-function objects to the newly created
module based upon the table (of type struct methodlist) that was
passed as its second parameter.  The function initmodule() returns a
pointer to the module object that it creates, but this is unused here.
It aborts with a fatal error if the module could not be initialized
satisfactorily.


\section{Calling the module initialization function}

There is one more thing to do: telling the Python module to call the
initfoo() function when it encounters an 'import foo' statement.
This is done in the file config.c.  This file contains a table mapping
module names to parameterless void function pointers.  You need to add
a declaration of initfoo() somewhere early in the file, and a line
saying

\begin{verbatim}
    {"foo",     initfoo},
\end{verbatim}

to the initializer for inittab[].  It is conventional to include both
the declaration and the initializer line in preprocessor commands
\verb\#ifdef USE_FOO\ / \verb\#endif\, to make it easy to turn the foo
extension on or off.  Note that the Macintosh version uses a different
configuration file, distributed as configmac.c.  This strategy may be
extended to other operating system versions, although usually the
standard config.c file gives a pretty useful starting point for a new
config*.c file.

And, of course, I forgot the Makefile.  This is actually not too hard,
just follow the examples for, say, AMOEBA.  Just find all occurrences
of the string AMOEBA in the Makefile and do the same for FOO that's
done for AMOEBA...

(Note: if you are using dynamic loading for your extension, you don't
need to edit config.c and the Makefile.  See "./DYNLOAD" for more info
about this.)


\section{Calling Python functions from C}

The above concentrates on making C functions accessible to the Python
programmer.  The reverse is also often useful: calling Python
functions from C.  This is especially the case for libraries that
support so-called "callback" functions.  If a C interface makes heavy
use of callbacks, the equivalent Python often needs to provide a
callback mechanism to the Python programmer; the implementation may
require calling the Python callback functions from a C callback.
Other uses are also possible.

Fortunately, the Python interpreter is easily called recursively, and
there is a standard interface to call a Python function.  I won't
dwell on how to call the Python parser with a particular string as
input -- if you're interested, have a look at the implementation of
the "-c" command line option in pythonmain.c.

Calling a Python function is easy.  First, the Python program must
somehow pass you the Python function object.  You should provide a
function (or some other interface) to do this.  When this function is
called, save a pointer to the Python function object (be careful to
INCREF it!) in a global variable -- or whereever you see fit.
For example, the following function might be part of a module
definition:

\begin{verbatim}
    static object *my_callback;

    static object *
    my_set_callback(dummy, arg)
        object *dummy, *arg;
    {
        XDECREF(my_callback); /* Dispose of previous callback */
        my_callback = arg;
        XINCREF(my_callback); /* Remember new callback */
        /* Boilerplate for "void" return */
        INCREF(None);
        return None;
    }
\end{verbatim}

Later, when it is time to call the function, you call the C function
call_object().  This function has two arguments, both pointers to
arbitrary Python objects: the Python function, and the argument.  The
argument can be NULL to call the function without arguments.  For
example:

\begin{verbatim}
    object *result;
    ...
    /* Time to call the callback */
    result = call_object(my_callback, (object *)NULL);
\end{verbatim}

call_object() returns a Python object pointer: this is
the return value of the Python function.  call_object() is
"reference-count-neutral" with respect to its arguments, but the
return value is "new": either it is a brand new object, or it is an
existing object whose reference count has been incremented.  So, you
should somehow apply DECREF to the result, even (especially!) if you
are not interested in its value.

Before you do this, however, it is important to check that the return
value isn't NULL.  If it is, the Python function terminated by raising
an exception.  If the C code that called call_object() is called from
Python, it should now return an error indication to its Python caller,
so the interpreter can print a stack trace, or the calling Python code
can handle the exception.  If this is not possible or desirable, the
exception should be cleared by calling err_clear().  For example:

\begin{verbatim}
    if (result == NULL)
        return NULL; /* Pass error back */
    /* Here maybe use the result */
    DECREF(result); 
\end{verbatim}

Depending on the desired interface to the Python callback function,
you may also have to provide an argument to call_object().  In some
cases the argument is also provided by the Python program, through the
same interface that specified the callback function.  It can then be
saved and used in the same manner as the function object.  In other
cases, you may have to construct a new object to pass as argument.  In
this case you must dispose of it as well.  For example, if you want to
pass an integral event code, you might use the following code:

\begin{verbatim}
    object *argument;
    ...
    argument = newintobject((long)eventcode);
    result = call_object(my_callback, argument);
    DECREF(argument);
    if (result == NULL)
        return NULL; /* Pass error back */
    /* Here maybe use the result */
    DECREF(result);
\end{verbatim}

Note the placement of DECREF(argument) immediately after the call,
before the error check!  Also note that strictly spoken this code is
not complete: newintobject() may run out of memory, and this should be
checked.

In even more complicated cases you may want to pass the callback
function multiple arguments.  To this end you have to construct (and
dispose of!) a tuple object.  Details (mostly concerned with the
errror checks and reference count manipulation) are left as an
exercise for the reader; most of this is also needed when returning
multiple values from a function.

XXX TO DO: explain objects and reference counting.
XXX TO DO: defining new object types.


\section{Format strings for getargs()}

The getargs() function is declared in "modsupport.h" as follows:

\begin{verbatim}
    int getargs(object *arg, char *format, ...);
\end{verbatim}

The remaining arguments must be addresses of variables whose type is
determined by the format string.  For the conversion to succeed, the
`arg' object must match the format and the format must be exhausted.
Note that while getargs() checks that the Python object really is of
the specified type, it cannot check that the addresses provided in the
call match: if you make mistakes there, your code will probably dump
core.

A format string consists of a single `format unit'.  A format unit
describes one Python object; it is usually a single character or a
parenthesized string.  The type of a format units is determined from
its first character, the `format letter':

's'	(string)
	The Python object must be a string object.  The C argument
	must be a char** (i.e., the address of a character pointer),
	and a pointer to the C string contained in the Python object
	is stored into it. If the next character in the format string
	is \verb\'#'\, another C argument of type int* must be present, and
	the length of the Python string (not counting the trailing
	zero byte) is stored into it.

'z'	(string or zero, i.e., NULL)
	Like 's', but the object may also be None.  In this case the
	string pointer is set to NULL and if a \verb\'#'\ is present the size
	it set to 0.

'b'	(byte, i.e., char interpreted as tiny int)
	The object must be a Python integer.  The C argument must be a
	char*.

'h'	(half, i.e., short)
	The object must be a Python integer.  The C argument must be a
	short*.

'i'	(int)
	The object must be a Python integer.  The C argument must be
	an int*.

'l'	(long)
	The object must be a (plain!) Python integer.  The C argument
	must be a long*.

'c'	(char)
	The Python object must be a string of length 1.  The C
	argument must be a char*.  (Don't pass an int*!)

'f'	(float)
	The object must be a Python int or float.  The C argument must
	be a float*.

'd'	(double)
	The object must be a Python int or float.  The C argument must
	be a double*.

'S'	(string object)
	The object must be a Python string.  The C argument must be an
	object** (i.e., the address of an object pointer).  The C
	program thus gets back the actual string object that was
	passed, not just a pointer to its array of characters and its
	size as for format character 's'.

'O'	(object)
	The object can be any Python object, including None, but not
	NULL.  The C argument must be an object**.  This can be used
	if an argument list must contain objects of a type for which
	no format letter exist: the caller must then check that it has
	the right type.

'('	(tuple)
	The object must be a Python tuple.  Following the '('
	character in the format string must come a number of format
	units describing the elements of the tuple, followed by a ')'
	character.  Tuple format units may be nested.  (There are no
	exceptions for empty and singleton tuples; "()" specifies an
	empty tuple and "(i)" a singleton of one integer.  Normally
	you don't want to use the latter, since it is hard for the
	user to specify.


More format characters will probably be added as the need arises.  It
should be allowed to use Python long integers whereever integers are
expected, and perform a range check.  (A range check is in fact always
necessary for the 'b', 'h' and 'i' format letters, but this is
currently not implemented.)


Some example calls:

\begin{verbatim}
    int ok;
    int i, j;
    long k, l;
    char *s;
    int size;

    ok = getargs(args, "(lls)", &k, &l, &s); /* Two longs and a string */
        /* Possible Python call: f(1, 2, 'three') */
    
    ok = getargs(args, "s", &s); /* A string */
        /* Possible Python call: f('whoops!') */

    ok = getargs(args, ""); /* No arguments */
        /* Python call: f() */
    
    ok = getargs(args, "((ii)s#)", &i, &j, &s, &size);
        /* A pair of ints and a string, whose size is also returned */
        /* Possible Python call: f(1, 2, 'three') */

    {
        int left, top, right, bottom, h, v;
        ok = getargs(args, "(((ii)(ii))(ii))",
                 &left, &top, &right, &bottom, &h, &v);
                 /* A rectangle and a point */
                 /* Possible Python call:
                    f( ((0, 0), (400, 300)), (10, 10)) */
    }
\end{verbatim}

Note that a format string must consist of a single unit; strings like
\verb\'is'\ and \verb\'(ii)s#'\ are not valid format strings.  (But
\verb\'s#'\ is.)


The getargs() function does not support variable-length argument
lists.  In simple cases you can fake these by trying several calls to
getargs() until one succeeds, but you must take care to call
err_clear() before each retry.  For example:

\begin{verbatim}
    static object *my_method(self, args) object *self, *args; {
        int i, j, k;

        if (getargs(args, "(ii)", &i, &j)) {
            k = 0; /* Use default third argument */
        }
        else {
            err_clear();
            if (!getargs(args, "(iii)", &i, &j, &k))
                return NULL;
        }
        /* ... use i, j and k here ... */
        INCREF(None);
        return None;
    }
\end{verbatim}

(It is possible to think of an extension to the definition of format
strings to accomodate this directly, e.g., placing a '|' in a tuple
might specify that the remaining arguments are optional.  getargs()
should then return 1 + the number of variables stored into.)


Advanced users note: If you set the `varargs' flag in the method list
for a function, the argument will always be a tuple (the `raw argument
list').  In this case you must enclose single and empty argument lists
in parentheses, e.g., "(s)" and "()".


\section{The mkvalue() function}

This function is the counterpart to getargs().  It is declared in
"modsupport.h" as follows:

\begin{verbatim}
    object *mkvalue(char *format, ...);
\end{verbatim}

It supports exactly the same format letters as getargs(), but the
arguments (which are input to the function, not output) must not be
pointers, just values.  If a byte, short or float is passed to a
varargs function, it is widened by the compiler to int or double, so
'b' and 'h' are treated as 'i' and 'f' is treated as 'd'.  'S' is
treated as 'O', 's' is treated as 'z'.  \verb\'z#'\ and \verb\'s#'\
are supported: a second argument specifies the length of the data
(negative means use strlen()).  'S' and 'O' add a reference to their
argument (so you should DECREF it if you've just created it and aren't
going to use it again).

If the argument for 'O' or 'S' is a NULL pointer, it is assumed that
this was caused because the call producing the argument found an error
and set an exception.  Therefore, mkvalue() will return NULL but won't
set an exception if one is already set.  If no exception is set,
SystemError is set.

If there is an error in the format string, the SystemError exception
is set, since it is the calling C code's fault, not that of the Python
user who sees the exception.

Example:

\begin{verbatim}
    return mkvalue("(ii)", 0, 0);
\end{verbatim}

returns a tuple containing two zeros.  (Outer parentheses in the
format string are actually superfluous, but you can use them for
compatibility with getargs(), which requires them if more than one
argument is expected.)

\section{Reference counts}

Here's a useful explanation of INCREF and DECREF by Sjoerd Mullender.

Use XINCREF or XDECREF instead of INCREF/DECREF when the argument may
be NULL.

The basic idea is, if you create an extra reference to an object, you
must INCREF it, if you throw away a reference to an object, you must
DECREF it.  Functions such as newstringobject, newsizedstringobject,
newintobject, etc. create a reference to an object.  If you want to
throw away the object thus created, you must use DECREF.

If you put an object into a tuple, list, or dictionary, the idea is
that you usually don't want to keep a reference of your own around, so
Python does not INCREF the elements.  It does DECREF the old value.
This means that if you put something into such an object using the
functions Python provides for this, you must INCREF the object if you
want to keep a separate reference to the object around.  Also, if you
replace an element, you should INCREF the old element first if you
want to keep it.  If you didn't INCREF it before you replaced it, you
are not allowed to look at it anymore, since it may have been freed.

Returning an object to Python (i.e., when your module function
returns) creates a reference to an object, but it does not change the
reference count.  When your module does not keep another reference to
the object, you should not INCREF or DECREF it.  When you do keep a
reference around, you should INCREF the object.  Also, when you return
a global object such as None, you should INCREF it.

If you want to return a tuple, you should consider using mkvalue.
Mkvalue creates a new tuple with a reference count of 1 which you can
return.  If any of the elements you put into the tuple are objects,
they are INCREFfed by mkvalue.  If you don't want to keep references
to those elements around, you should DECREF them after having called
mkvalue.

Usually you don't have to worry about arguments.  They are INCREFfed
before your function is called and DECREFfed after your function
returns.  When you keep a reference to an argument, you should INCREF
it and DECREF when you throw it away.  Also, when you return an
argument, you should INCREF it, because returning the argument creates
an extra reference to it.

If you use getargs() to parse the arguments, you can get a reference
to an object (by using "O" in the format string).  This object was not
INCREFfed, so you should not DECREF it.  If you want to keep the
object, you must INCREF it yourself.

If you create your own type of objects, you should use NEWOBJ to
create the object.  This sets the reference count to 1.  If you want
to throw away the object, you should use DECREF.  When the reference
count reaches 0, the dealloc function is called.  In it, you should
DECREF all object to which you keep references in your object, but you
should not use DECREF on your object.  You should use DEL instead.

\chapter{Embedding Python in another application}

Embedding Python is similar to extending it, but not quite.  The
difference is that when you extend Python, the main program of the
application is still the Python interpreter, while of you embed
Python, the main program may have nothing to do with Python --
instead, some parts of the application occasionally call the Python
interpreter to run some Python code.

So if you are embedding Python, you are providing your own main
program.  One of the things this main program has to do is initialize
the Python interpreter.  At the very least, you have to call the
function initall().  There are optional calls to pass command line
arguments to Python.  Then later you can call the interpreter from any
part of the application.

There are several different ways to call the interpreter: you can pass
a string containing Python statements to run_command(), or you can
pass a stdio file pointer and a file name (for identification in error
messages only) to run_script().  You can also call the lower-level
operations described (partly) in the file \verb\<pythonroot>/misc/EXTENDING\
to construct and use Python objects.

A simple demo of embedding Python can be found in the directory
\verb\<pythonroot>/embed/\.

\section{Using C++}

It is also possible to embed Python in a C++ program; how this is done
exactly will depend on the details of the C++ system used; in general
you will need to write the main program in C++, enclosing the include
files in \verb\"extern "C" { ... }"\, and compile and link this with
the C++ compiler.  (There is no need to recompile Python itself with
C++.)

\input{ext.ind}

\end{document}